Interactions between the complement and coagulation systems will be defined via both structural and functional studies. The N-terminal amino acid sequence of properdin factor D has been determined and shown to be homologous with other complement and coagulation proteases. Further sequence data will be obtained, and will be directed toward determining the sequence of the active enzymatic site, and toward regions of the molecule which cross-react immunochemically with other proteases. Factor D has been shown to specifically inhibit thrombin induced platelet aggregation; this inhibition is mediated via competitive inhibition of thrombin binding to the platelet surface. Factor D binding to platelets will be comletely characterized and its effects upon platelet release will be evaluated.